1326 WATER TREATMENT
Granular Activated Carbon (GAC)The removal of VOC’s through adsorption involves passing
the contaminated water through a medium of adsorbent, such
as activated carbon, where the VOC’s will adhere (stick)
to its surface. Adsorbates which could possibly be used to
remove VOC’s from groundwater include granular activated
carbon (GAC) and powdered activated carbon (PAC). GAC
exhibits a wide range of effectiveness in adsorbing various
compounds and generally tends to adsorb high-molecular
weight compounds more readily than low-molecular weight
substances such as VOC’s. However, GAC is currently the
best available adsorbent for the removal of VOC’s.
Powdered activated carbon has been used traditionally
for the removal of trace organics associated with causing
taste and odors in drinking water. PAC typically requires
coagulation and sedimentation facilities to be effective and
is not normally used for groundwater treatment.
GAC has a spectrum of effectiveness like aeration; how-
ever, the process is more complicated and water quality
can have an influence on performance. The adsorption of
VOC’s can be affected by the amount of background organic
carbon, generally measured as total organic carbon. High
background organic content can result in lower adsorption
capacity.
GAC contractors also require regeneration or replacement
when the material becomes saturated with contaminants. The
life of the GAC is dependent upon the concentrations of the
contaminants present, the flow rate through the media and
the required effluent concentrations. GAC contractors have a
reported removal efficiency of 99%. A schematic of a typical
GAC facility is shown in Figure B.
A combination of aeration-adsorption can also be a
highly effective method of reducing VOC levels to very
low concentrations. This combination is quite attractive
when several different types of contaminants are present.
However, the corresponding cost of treatment increases
dramatically.VOC Treatment with Granular Activated Carbon
(GAC)GAC treatment can employ either a gravity or pressure
system. The gravity disrepair is generally used in surface
water treatment plants and operates in a manner similar to a
gravity sand filter.
In groundwater treatment systems, a pressure disrepair
(contactor) is generally used and involves a pressurized
vessel which can accommodate flow rates at high pres-
sures and allow direct discharge to an existing distribution
system.
Numerous GAC contactors are currently in use for the
removal of VOCs and a significant amount of data is avail-
able on this form treatment.
GAC contactors would involve vertical steel pressure
vessels which would allow the raw water to enter the top of
the vessel and pass downward through the carbon bed.The treated water is collected at the bottom of the
vessel utilizing a header-lateral arrangement or a bottom
plate with nozzles. The collected water would then be dis-
infected and discharged to the distribution system for con-
sumption.
Since the contactor is essentially a filter, the vessel would
be equipped with backwashing facilities. The carbon filters
would not require frequent backwashing. The contactor back-
wash waste would be disposed of by discharging to the nearby
holding pond and subsequently to the sanitary sewer system.
Connections would also be provided to readily remove the
spent carbon and to readily install the new material.
As a general rule, aeration is most effective with low-
molecular weight, highly volatile substances, while adsorp-
tion works best with high-molecular weight compounds with
a low solubility. The selection of the treatment alternative is
based on many factors such as the contaminant(s), concentra-
tions of contaminants, groundwater quality, site constraints,
pumping system configuration as well as other factors.
Air stripping facilities, by their nature, require the exist-
ing pumping facilities to be modified as well as the need to
install a second pumping system. Existing pumping systems
must be modified to produce less head (pressure) in order to
direct water to the stripping tower. The tower will dissipate
the energy provided by the well pump as the water passes
through the tower and into the clearwell below. From the
clearwell, the water must be repumped to the water system
for use. As such, the economic feasibility of an air-stripping
facility must account not only for the capital and operating
expenses of the stripping facility, but must also account for
modifications to existing well pump(s) and the costs associ-
ated with repumping the water supply for use.
Unlike an air-stripping facility, a pressurized carbon
contactor does not utilize mechanical equipment as part of
the treatment process. Most often, existing pumping facili-
ties may remain unchanged if a small head loss (which
would result in a slightly reduced flowrate) can be tolerated.
After water is pumped through the contactor, it is discharged
directly to the distribution system for use. As such, GAC
facilities have a major advantage over air strippings due
to ease of operation and the ability to discharge directly to
existing systems without repumping.
There is a move toward privatization of public water
(and wastewater) operations. There are advantages and dis-
advantages associated with this. The Contractor assumes
responsibility for operating results. If a plant has a staff that
is too large or inadequately trained, a for-profit operation can
be expected to introduce greater efficiency in operation. At
times, public office holders have introduced privatization to
show a better municipal financial picture than is the actual
case. The Contractor may not practice proper maintenance
or may try to operate with a staff that is too small. This may
be reflected in poor operating results and the public agency
charged with oversight can be expected to take action if the
public health is threatened. Each case is unique and each
decision to privatize must be evaluated taking into account all
pertinent factors.C023_006_r03.indd 1326C023_006_r03.indd 1326 11/18/2005 1:32:37 PM11/18/2005 1:32:37 PM